The epitaxial growth of Pt and Ru deposits by spontaneous, as well as by dynamic, electrodeposition onto Ru(0001) and Pt(111), respectively, have been studied by reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). For the Pt deposit on Ru(0001), at submonolayer range, it preferably grows compressed commensurate bilayer thick islands on Ru(0001). This is the first time that RHEED observation of the onset of Pt twinning occurs in ca. 2-3 layer thick islands on Ru at room temperature, at which the surface strain due to the 2.5% lattice mismatch of Pt and Ru remains intact. For multilayer thick islands (>6？ML) ordered reflection twins (diameter of 3？nm) develop and are embedded in a (111) matrix with an incoherent (11-2) twin plane normal to Ru(0001) and aligned with their [？110] direction parallel to the [11-20] Ru(0001) substrate direction. For the Ru deposit on Pt(111), at 0.2 ML a strained ( ) monoatomic layer is formed due to the 2.5% lattice mismatch of Ru and Pt. Increasing the coverage up to 0.64, the second Ru layer is found to relieve the strain in the first layer, giving rise to dislocations and Ru relaxes to its bulk lattice constant. Multilayers of Ru (>1？ML) result in (0001) nanocluster formation aligned with its [11-20] direction parallel to the [？110] Pt(111) substrate direction. 1. Introduction Considerable attention has been given to the growth of mixed metals, which enables the preparation of materials with particular chemical properties. Apart from evaporation in UHV [1, 2], also spontaneous [3–5] or electrochemical deposition [6–8] from electrolyte solution has been used most often to prepare the modified catalyst surfaces. This interest stems from the possibility of preparing modified electrodes as model systems to probe the reactivity at the interfaces. The most widespread and promising bimetal-catalysts for fuel cell (FC) applications employ dispersed bimetal nanoparticles as electrocatalysts, usually a platinum based alloy. Bimetallic Pt catalysts, mainly Pt-Ru [9–11], but also Pt-Sn [12], Pt-Mo [13], and Pt-Os [14], have been intensively studied in order to achieve the optimal electrooxidation of methanol via the so-called bifunctional mechanism [15, 16]. Spontaneous deposition has been used to prepare the Ru-modified Pt(111) [7, 17] as well as Pt/Ru(0001) surfaces [4, 18, 19]. Remarkable enhancements of the electrocatalytic activity towards CO, methanol, and formic acid oxidation have been found with both Ru-modified Pt electrode and Pt-modified Ru electrodes [4, 18, 19]. The spontaneously
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